[Doc] Add docstring for MCTSForest.extend

ghstack-source-id: dbef5e48ea55db6ba7867e1b24eb4711ad08af61
Pull Request resolved: #2795

Author: kurtamohler

torchrl/data/map/tree.py

@@ -1003,6 +1003,33 @@ def _make_node_map(self, source, dest):
             self.max_size = self.data_map.max_size
 
     def extend(self, rollout, *, return_node: bool = False):
+        """Add a rollout to the forest.
+
+        Nodes are only added to a tree at points where rollouts diverge from
+        each other and at the endpoints of rollouts.
+
+        If there is no existing tree that matches the first steps of the
+        rollout, a new tree is added. Only one node is created, for the final
+        step.
+
+        If there is an existing tree that matches, the rollout is added to that
+        tree. If the rollout diverges from all other rollouts in the tree at
+        some step, a new node is created before the step where the rollouts
+        diverge, and a leaf node is created for the final step of the rollout.
+        If all of the rollout's steps match with a previously added rollout,
+        nothing changes.  If the rollout matches up to a leaf node of a tree but
+        continues beyond it, that node is extended to the end of the rollout,
+        and no new nodes are created.
+
+        Args:
+            rollout (TensorDict): The rollout to add to the forest.
+            return_node (bool, optional): If True, the method returns the added
+                node. Default is ``False``.
+
+        Returns:
+            Tree: The node that was added to the forest. This is only
+                returned if ``return_node`` is True.
+        """
         source, dest = (
             rollout.exclude("next").copy(),
             rollout.select("next", *self.action_keys).copy(),